1--Field of the invention
This invention in general relates to a conversion of two binary signals into one bipolar signal with positive or negative pulses, or marks, corresponding alternately to a first logic state of the two binary signals.
2--Description of the prior art
More particularly, the invention deals with a binary-to-bipolar converter located at the output of a transmitting part, called transmitter, of a terminal connected to a distributed transmission line in a local area network. In such a network many terminals are connected to the transmission line and must therefore have a high fan-out. This condition means that the converter must have a high output impedance during the time intervals between bipolar signal pulses and have a low output impedance during transmission of the bipolar signal pulses. When the converteris at a high impedance state, its output is considered to be "disconnected from the line" and is often said to be with "open collector".
A known converter meeting these impedance conditions comprises, on the input side, two CMOS technology tristate transfer gates and, on the output side, a balanced differential transformer. The gates have inputs raised at a positive DC voltage, control terminals receiving respectively the binary signals, and outputs transmitting the respective positive and negative pulses to two primary half-windings of the transformer.
The pulses of the bipolar signal transmitted by this known converter have rear edges, i.e., transitions from the low impedance state to the high impedance state, which vary slowly. The slow variation of the rear edges is due to the energy stored in the transformer during the pulses, energy which slowly flows out into the line load throughout the rear fronts of the bits at the first logic state, generally high of the binary signals. The fall time of the pulse rear fronts is all the greater as the load impedance is high, and disturbs the reception of the bipolar signal accordingly.